As was explained in U.S. application Ser. No. 258,675, filed Oct. 17, 1988, now abandoned, in the names of G. H. Beall and C. J. Quinn under the title ZINC-CONTAINING PHOSPHATE GLASSES, extensive research has been conducted to discover inorganic glass compositions demonstrating low transformation or transition temperatures (T.sub.g), which property renders it possible to undertake glass batch melting and forming operations at low temperatures. And, as further explained there, because silicate-based glass compositions typically exhibit transition temperatures substantially greater than 450.degree. C. and forming temperatures considerably higher than 800.degree. C., the search for glasses having low transition temperatures has commonly led to the use of B.sub.2 O.sub.3 - and/or P.sub.2 O.sub.5 -based compositions. Unfortunately, however, phosphate-based glass compositions customarily demonstrate much poorer chemical durability than silicate-containing glasses, with the poor durability of those glasses becoming more notable as the transition temperature of the glass is reduced. As conventionally defined, the transition temperature of a glass is considered to be the temperature at which increases in specific heat and coefficient of thermal expansion occur which are accompanied by a sharp drop in viscosity.
Consequently, the principal objective of Ser. No. 258,675 was to devise glass compositions manifesting transition temperatures below 450.degree. C., preferably below 350.degree. C., with working temperatures (temperatures at which the glass displays a viscosity of about 10.sup.4 -10.sup.7 poises) below 500.degree. C., preferably between about 350.degree.-450.degree. C., and which demonstrate excellent resistance to chemical attack when exposed to boiling water and mild aqueous alkaline solutions. To achieve that objective, glasses were disclosed consisting essentially, expressed in terms of mole percent on the oxide basis, of 10-35% R.sub.2 O, wherein R.sub.2 O consists of at least two alkali metal oxides in the indicated proportions of 0-25% Li.sub.2 O, 0-25% Na.sub.2 O, and 0-25% K.sub.2 O, 12-55% ZnO, 28-40% P.sub.2 O.sub.5, and, optionally, up to 35% RO, wherein RO consists of 0-35% PbO and 0-35% SnO. It was observed that at least two alkali metal oxides must be present inasmuch as the inclusion of Li.sub.2 O alone risks the possible development of devitrification in the glass; Na.sub.2 O alone reduces the chemical durability of the glass or renders it difficult to melt; and K.sub.2 O alone raises the T.sub.g of the glass to unacceptable levels. The preferred glasses contained 0.75-6% Al.sub.2 O.sub.3 and/or 1-8% B.sub.2 O.sub.3, the total Al.sub.2 O.sub.3 +B.sub.2 O.sub.3 not exceeding 8%. The most preferred glasses consisted essentially of 12-25% R.sub.2 O, wherein R.sub.2 O consists of at least two alkali metal oxides in the indicated proportions of 3-12% Li.sub.2 O, 4-13% Na.sub.2 O, and 0-12% K.sub.2 O, 30-49% ZnO, 0-10% SnO, 1-3.5% Al.sub.2 O.sub.3, 30-36% P.sub.2 O.sub.5, and 0-5% CaO+MgO+F, wherein the sum of R.sub.2 O+Al.sub.2 O.sub.3 +ZnO+P.sub.2 O.sub.5 with, optionally, SnO totals at least 85%.
Although an exact conversion of composition intervals expressed in terms of mole percent to ranges defined in terms of weight percent is not mathematically possible, Ser. No. 258,675 provided the following approximate values in weight percent of the broad operable composition area: 5-25% R.sub.2 O, wherein R.sub.2 O consists of at least two alkali metal oxides in the indicated proportions of 0-10% Li.sub.2 O, 0-15% Na.sub.2 O, and 0-20% K.sub.2 O, 6-45% ZnO, 30-57% P.sub.2 O.sub.5, and, optionally, up to 55% RO, wherein RO consists of 0-50% PbO and 0-40% SnO. In the preferred compositions 0.75-6% Al.sub.2 O.sub.3 and/or 1-6% B.sub.2 O.sub.3 will be present, the sum of the two components not exceeding 6%.
Whereas the glasses disclosed in Ser. No. 258,675 demonstrated exceptional chemical durability, especially when it is appreciated that those glasses exhibited T.sub.g values below 450.degree. C., research continued to discover glass compositions displaying the physical properties of those glasses, but wherein the chemical durability could be further enhanced.
Accordingly, the principal objective of the present invention was to develop glasses exhibiting transition temperatures below 450.degree. C., preferably below 350.degree. C., and with working temperatures below 500.degree. C., preferably between about 350.degree.-450.degree. C., but which demonstrate levels of chemical durability, when contacted with mild aqueous solutions, at least ten times better than those exhibited by the glasses of Ser. No. 258,675.